Breakup of pancake droplets flowing through a microfluidic constriction

Due to its significance in oil recovery and droplet microfluidics, breakup of spherical droplets in a constriction (i.e., snap-off) is extensively studied. However, rectangular channels with large aspect ratios are commonly used in real applications, in which droplets usually present pancake shape. Since capillary pressures governing droplet's snap-off depend strongly on droplet shape, we hypothesize that pancake droplets may exhibit different snap-off behaviors from spherical droplets. Via microfluidic experiments, we demonstrate that pancake droplets indeed present new snap-off behaviors in three aspects: (1) they snap off at high capillary numbers; (2) the interface breaks up at the rear meniscus instead of the front one; (3) the size of daughter droplets is dependent of mother droplet size instead of independent for spherical droplets. Besides, we reveal the underlying mechanism, i.e., pancake droplets show much lower velocity in the constriction than spherical droplets, via analyzing capillary pressures at the droplet's two ends.